Liquid conductivity measurement systems are used for the measurement of conductivity of water and aqueous or non-aqueous solutions in environmental, medical, industrial, and other applications where an indication of the ionic content of the liquid is required.
Liquid conductivity is measured in a variety of contexts to provide a relatively inexpensive parameter that can be sometimes related to bulk ionic concentration. In situations where a single type of ion is present, the conductivity can actually be related to specific ionic concentration. Even in situations where a number of different ionic compounds are present, the measurement of bulk liquid conductivity can still provide very useful information. Accordingly, there has been widespread adoption and utilization of conductivity measurement by the industry for a variety of different purposes. Given the variety of different applications for such systems, it is expected that some will be employed to provide conductivity measurements for low-conductivity liquids, while others will be employed to provide conductivity measurements for high-conductivity liquids.
Typically, contact-based conductivity measurement systems include a conductivity cell and an associated conductivity meter. FIG. 1 illustrates such a system. A conductivity meter generates an AC voltage that is applied to electrodes of the conductivity cell. The meter then senses the resultant current that flows between the electrodes of the cell. This current is generally a function of the conductivity of the liquid to which the cell is exposed and is used to determine the conductance.
The amount of current that flows between the electrodes depends not only the solution conductivity, but also on the length, surface area, and geometry of the sensor electrodes. The probe constant (also called sensor constant or cell constant) is a measure of the current response of a sensor to a conductive solution, due to the sensor's dimensions and geometry. Its units are cm−1 (length divided by area), and the probe constant necessary for a given conductivity range is based on the particular conductivity analyzer's measuring circuitry. Probe constants can vary from 0.01 cm−1 to 50 cm−1 and, in general, the higher the conductivity, the larger the probe constant necessary.
While contacting conductivity based techniques can measure down to pure water conductivity, their primary drawback has been that the sensor itself is susceptible to coating and corrosion, which can drastically lower the reading. In strongly conductive solutions, there can also be polarization effects, which result in non-linearity in the measurement. Providing a contacting-type conductivity sensor that could better measure high-conductivity solutions would allow such contacting-type sensors to be used in a greater variety of applications.